Method for making pyroelectric sensors comprising a thin pyroelectric film requiring an electric polarisation

ABSTRACT

Method for manufacturing a plurality of pyroelectric sensors by forming a thin pyroelectric film or layer on one face of a silicon wafer or substrate, wherein electric polarisation of this film is provided between lower and upper electrodes defining pixels forming these sensors. In order to protect the wafer in the event of a short-circuit between two electrodes of a pixel, resistors are arranged in series with the lower or upper electrodes by connecting these electrodes to each other by subsets in order to carry out the electric polarisation. Once this polarisation has been carried out, the electric connections connecting the upper or lower electrodes are removed to allow each pixel to supply an elementary electric signal when the sensor is operating. In order to minimise the risk of short-circuits and in order to reduce the stray capacity of the electrodes, the upper and lower electrodes of the pixels are structured. In the case of intersections of the connecting paths in projection onto the general plane of the wafer, these intersections are preferably provided between the electrodes and the respective resistors.

[0001] The present invention concerns a method for manufacturingpyroelectric sensors by forming a thin pyroelectric film or layer on oneface of the same wafer or substrate, particularly made of silicon. Eachsensor is formed of several pixels each defined by a first electrode ofits own located on one face of the pyroelectric film and a secondelectrode located on the other face of said film.

[0002] Such pyroelectric sensors have several uses, particularly for gasspectrometry and thermal imagining. They are relatively inexpensive.However, for the pyroelectric film to work properly, it is generallynecessary for it to undergo electric polarisation, i.e. the applicationof an electric field between two electrodes which has the effect oforienting the dipoles of the pyroelectric film at least between thefirst and second electrodes of the pixels. By way of example, anelectric field of approximately 30 V is applied across the first andsecond electrodes of the pixels with a temperature of the pyroelectricfilm of between 110 and 170° C., for approximately 10 minutes. Aplurality of sensors is batch manufactured with technology similar tothat for the manufacture of integrated circuits. A silicon wafer of adiameter of 10 cm can contain several hundred sensors formed either oflinear networks of pixels or two-dimensional networks in the form ofpixel matrices.

[0003] A method for manufacturing such sensors has to allow at leastpixel subsets to be polarised simultaneously; pixel-by-pixelpolarisation having to be set aside for obvious reasons. Preferably, allthe pixels of the same wafer are polarised simultaneously so as toreduce the manufacturing costs and the manufacturing time of thesensors. In order to polarise at least a pixel subset, on the one handtheir upper electrodes and on the other hand their lower electrodes areelectrically connected to each other. Generally, either the upperelectrodes, or the lower electrodes are electrically connected to eachother permanently so as to be set at a common potential during operationof the sensor. However, the other electrodes have to be electricallyinsulated from each other so as to provide elementary electric signals.

[0004] In general, those skilled in the art provide for the lowerelectrodes to be formed by a metallic film defining a common electrodefor all the pixels. This allows manufacturing steps to be saved. Thus,the silicon wafer has a continuous metal film on which the thinpyroelectric film is deposited. The upper electrodes, whichgeometrically define the pixels, are deposited on this film.

[0005] In the aforementioned example, the upper electrodes of the sensorhave to be insulated from each other prior to its use, as mentionedhereinbefore. However, in order to simultaneously polarise at least apixel subset, temporary electric connections are formed between theupper electrodes. In other words, in order to carry out the polarisationthe electrodes provided to supply the elementary electric pixel signalsare also electrically connected. Thus, preferably, all these electrodeswill be connected at least by subsets connected to at least one electriccontact pad provided to carry out the polarisation.

[0006] However, such a method has a major drawback given that anyaccidental short-circuit between the electrodes of a pixel will cause avoltage drop for all the pixels of the subset concerned. Electricpolarisation thus becomes impossible and a whole wafer may be wasted.Given the number of sensors manufactured on the same wafer, such anevent causes considerable losses, which significantly increases the costprice of such sensors in industrial production.

[0007] The object of the present invention is to overcome theaforementioned drawback while allowing simultaneous electricpolarisation for at least pixel subsets of a sensor wafer beingmanufactured.

[0008] This object is achieved by the subject of the invention asdefined in claim 1 which concerns a method for manufacturing a pluralityof pyroelectric sensors wherein there is provided a step where electricconnections are formed between at least a subset of first electrodeseach forming an electrode belonging to a pixel and wherein electricresistors are formed, arranged such that each electrode of said subsetis series connected with one of said resistors.

[0009] As a result of the arrangement of the aforementioned electricresistors, a short-circuit at one pixel no longer causes a critical dropin the polarisation voltage applied during a polarisation step followingthe aforementioned step. Thus, it is possible to polarise the sensors ofa wafer properly even in the presence of one or several short-circuitsat certain pixels. The short-circuited pixels are of course inoperativein the operation of the manufactured sensors, but this problem remainslocalised only at the pixel that has been accidentally short-circuited.

[0010] So as to further reduce the risk of short-circuits anddeterioration of the sensors, within the scope of the invention andaccording to a preferred embodiment, it is provided to also structurethe electrodes intended to be connected to each other to a referencepotential. Thus, the pixels are defined by lower and upper electrodesbelonging to the pixels. This allows stray capacities formed by theelectrodes to be reduced. These second electrodes are connected at leastpartially to each other by second electric connections, i.e. they areelectrically connected at least by subsets preferably corresponding tothe previously mentioned subsets.

[0011] According to sensor manufacturing variant, the first and secondelectric connections respectively connecting upper and lower electrodesintersect in projection onto the general plane of the wafer. To preventany short-circuit between these first and second electric connections atthe intersections causing deterioration of part of the sensors or all ofthem by a polarisation voltage drop, the first and second connectionsare arranged such that the intersections are located between theresistors and the electrodes to which they are connected.

[0012] According to a particularly advantageous embodiment, well suitedfor the linear pixel networks or for matrices with two lines of pixels,the connections between the electrodes associated with the protectiveresistors are automatically broken when the sensors are diced.

[0013] Other embodiments and variants for implementing the methodaccording to the invention, in addition to particular advantages of theinvention will be described hereinafter using the following description,made with reference to the annexed drawing given by way of non-limitingexample and in which:

[0014]FIG. 1 shows partially and schematically a top view of apyroelectric sensor wafer being manufactured in accordance with a firstimplementation mode of the method according to the invention;

[0015]FIG. 2 is a cross-section along the line II-II of FIG. 1;

[0016]FIG. 3 is a partial schematic top view of a pyroelectric sensorwafer being manufactured in accordance with a second implementation modeof the method according to the invention; and

[0017]FIGS. 4 and 5 are cross-sections respectively along the linesIV-IV and V-V of FIG. 3.

[0018] A first implementation mode of the manufacturing method accordingto the invention will be described hereinafter with reference to FIGS. 1and 2. FIG. 1 partially shows a sensor 2 formed of a linear pixelnetwork 4. Each pixel is defined by the geometry of an upper electrode 6which belongs thereto. Each of these pixels is formed of a thinpyroelectric film 8 on the upper face of which is formed electrode 6 andon the lower face of which is provided a lower electrode 10 formed on amembrane 12. Under membrane 12, formed particularly of SiO₂ and/orSi₃N₄, there is provided a cavity 14 for thermally insulating eachpixel.

[0019] The sensors are formed on an upper face 16 of a silicon wafer 18.Thousands of pyroelectric sensors can be manufactured on this wafer.During manufacture of such sensors, it is possible to provide a lowerelectrode common to all the pixels of a sensor, or even common to allthe sensors and forming a continuous metallic film on the wafer.Likewise, pyroelectric film 8 can be common to the pixels of one sensor,or to all the sensors of the same wafer. However, for the purpose ofthermally insulating the pixels from each other, film 8 is generally atleast partially structured. In the case of FIG. 1, film 8 will beremoved particularly in the regions located between electrodes 6.Finally, an absorption film 20, for example of black platinum (not shownin FIG. 1) is deposited on the upper electrodes.

[0020] In order to polarise film 8 electrically between electrodes 6 and10 of each pixel, lower electrodes 10 and connected to each other byelectric connections 24 ending in at least one electric contact pad 26.Likewise, the upper electrodes are electrically connected to each otherby temporary electric connections formed of main paths 28 and secondarypaths 30 connecting each electrode 6 to a main path. These electricconnections are thus formed in accordance with the method of theinvention to allow at least one pixel subset to be simultaneouslypolarised. In order to do this, main paths 28 are connected to at leastone electric contact pad 32. It will be noted that for one wafer,generally several contact pads 26 and/or 32 are provided each connectinga subset of lower electrodes and/or upper electrodes of the sensorsbeing manufactured.

[0021] According to the invention, a resistor 36 is formed for eachpixel 4, series connected with the upper or lower electrode of saidpixel. In the example described here, resistors 36 are series connectedwith electrodes 6 and located along secondary paths 30. The electricconnections of electrodes 6 to pad 32 are provided firstly to allow anelectric polarisation step of pyroelectric film 8 to be carried out byapplying a voltage between the lower and upper electrodes, via contactpads 26 and 32. Secondly, they are provided for the deposition ofabsorption film 20 by an electrochemical process. By way of example,resistor 36 has a value of the order of 10 kΩ.

[0022] As was already mentioned in the introductory part of the presentdescription, resistor 36 is used firstly to protect the sensor waferbeing manufactured during the electric polarisation step. Indeed, if ashort-circuit occurs in a pixel between electrodes 4 and 6, resistor 36greatly limits the stray current and prevents the polarisation voltagedropping; which would otherwise cause the loss of at least a part of thewafer and thus thousands of sensors. Those skilled in the art will beable to select the range of suitable values for resistor 36. Secondly,these resistors 36 are used to define a substantially equal impedancefor each upper electrode 6, which is advantageous for allowingdeposition of absorption film 20 in a uniform manner over all of theupper electrodes.

[0023] An individual contact pad 38 is provided for each pixel, arrangedbetween the upper electrode and the resistor.

[0024] In order to reduce the risk of short-circuits between the lowerand upper metallisations with respect to film 8, in the case of thepresent invention lower electrode 10 is also structured and theseelectrodes are connected by connecting paths 24. For the purpose ofprotecting the plurality of sensors in the event of a short-circuitbetween the lower and upper electric connections, the intersections ofthese connections in projection onto the general plane 42 of the waferpreferably occur between electrodes 6 and the respective resistors 38.Thus, if an accidental short-circuit occurs between connections 24 and30, resistors 36 protect the rest of the pixels and in particular thesensors not concerned by this localised deterioration.

[0025] Main paths 28 are castellated and, when a sensor is separatedfrom the wafer, said wafer is diced along dotted lines 44. Thus, duringdicing, the electric connections between the upper electrodes areautomatically cut such that these electrodes are then electricallyinsulated from each other. Pads 38 are used to receive elementary pixelsignals and it will be noted that resistor 36 is located on the otherside of electrode 6 relative to pad 38 such that it is inactive when thesensor operates.

[0026] It will be noted that resistors 36 are made here in thinessentially horizontal film(s) and are located on the side of upper face16 of wafer 18. These resistors can be arranged in one or several layersin accordance with known techniques.

[0027] In the case of sensors arranged in a matrix formed of two linesof pixels, it is possible to arrange the various elements in a similarmanner to FIG. 1, each of the lines of pixels having its resistorsarranged on the side opposite to the other line, whereas in FIG. 1, theresistors are arranged alternately on one side of the pixels and theother. In the case of a pixel matrix having more than two lines, it ispossible to provide temporary electric connections partially arrangedbetween the pixels, particularly to reach the non-peripheral pixels. Inorder to save space and keep a high pixel density per surface unit, itis possible for large pixel matrices to arrange these electricconnections in a multilayered structure where different metallisationlevels are insulated from each other in accordance with knownmetallisation techniques from the semiconductor field. Such anembodiment ensures micromachining only on the side of upper face 16 ofwafer 18. However, the manufacturing steps are increased and the thermalinsulation of the pixels can be reduced if the electric connections andtheir insulating layers are not removed between the pixels. Anadvantageous solution for the electric connections necessary forpolarisation of relatively spread two-dimensional sensors is givenhereinafter.

[0028] With reference to FIGS. 3 and 4, a second implementation mode ofthe method according to the invention will be described hereinafter.FIG. 3 shows partially an arrangement of pixels in matrix form. Only 4pixels 54 are shown; but a sensor can be formed of a matrix havingseveral pixel lines and columns. The method described here can beapplied to sensors formed of a large number of pixels. The referencesalready described previously will not be described again here.

[0029] Pixels 54 are formed of lower 6 and upper 10 electrodes locatedon each side of a structured pyroelectric film 8. In order to protectfilm 12 during micromachining of electrode 10 and its connection arm 58by dry etching, a buffer layer or barrier layer 60 is provided abovefilm 12. Layer 60 is relatively thick and also forms a barrierpreventing interdiffusions between the pyroelectric film and membrane12. Moreover, it acts as an adhesion layer for the lower electrode. Itcan be formed particularly of ZrO₂, TiO₂ or other similar oxides,resistant to oxidation and to reactions with the pyroelectric film.

[0030] The electric connections between the upper electrodes areachieved by means of conductive vias connecting connection arms 56arranged on film 8 to paths 68 arranged on lower face 66 of wafer 18.The connections between the upper electrodes are thus achieved fromunderneath wafer or sensor concerned. Likewise, lower electrodes 10 areelectrically connected to each other by means of vias 70 verticallyconnecting arms 58 to conductive paths 72 located on rear face 66 ofwafer 18. Paths 68 and 72 are rectilinear and pass through wafer 18 toconnect upper and lower electrode rows. They have an electric contactpad at at least one end. The arrangement described hereinbefore allowssimultaneous polarisation of all of the pixels arranged on wafer 18 tobe carried out.

[0031] According to the invention, vias 64 or 70 form protectiveresistors for the wafer being manufactured during electric polarisationof the pixels. In the example given here, it is vias 70 which form theelectric protective resistors. After the polarisation step, the electricconnections between the lower electrodes are preferably removed byremoving paths 72 by a wet etching or dry etching step, by chemicalmechanical polishing (CMP) or by laser cutting. However, it may beadvantageous to keep paths 68 so as to be able to set the upperelectrodes of a sensor at a same electric potential. In order to collectthe elementary electric pixel signals, contact pads 78 are provided,located between electrode 10 and resistor 70. However, it is alsopossible to provide these pads 78 on the back face 66 of the wafer, forexample at the ends of vias 70 opening onto said back face. The use ofvias thus allows the problem of space requirement linked to thenecessary electric connections to be resolved, and allows the protectiveresistors to be advantageously arranged at the vias.

[0032] Those skilled in the art may also devise other variants withoutdeparting from the scope of the present invention. In particular, it ispossible to provide electric connections from the back face of the waferfor the lower electrodes, but to provide electric connections for theupper electrodes on the side of front face 16, i.e. above layer 60 andin particular above the pyroelectric film, as explained in the firstimplementation of the invention. In such a mixed embodiment, theresistors can be arranged either at the vias or at the metallisationlevel of the upper electrodes.

[0033] It will also be noted that electric resistors can be arranged onback face 66 of the wafer.

1. Method for manufacturing a plurality of pyroelectric sensors byforming a thin pyroelectric film (8) on one face of the same wafer (18),particularly made of silicon, defining a general plane (42), each sensorbeing formed of several pixels (4; 54) each defined by a first electrode(6) of its own located on one face of said pyroelectric film and asecond electrode (10) located on the other face of said film, thismethod being characterised in that at least the following successivesteps are provided: A) forming first electric connections (28; 30; 72)between at least a subset of first electrodes of said plurality ofsensors connecting these first electrodes to at least one electriccontact pad (32) and forming electric resistors (38; 70) arranged suchthat each first electrode of said at least one subset is seriesconnected with one of these resistors; B) electrically polarising saidpyroelectric film between the first and second electrodes by applying anelectric voltage across said at least one electric contact pad and saidsecond electrodes of the pixels corresponding to said at least onesubset of first electrodes; C) electrically insulating said first and/orsecond electrodes of at least one sensor from each other.
 2. Methodaccording to claim 1, characterised in that said second electrodes ofthe pixels are also formed by electrodes of their own that are locatedrespectively facing said first electrodes and connected at leastpartially to each other by second electric connections (34; 68). 3.Method according to claim 2, characterised in that said first and secondelectric connections intersect in projection onto the general plane ofsaid wafer, said second connections being arranged such that theseintersections occur between said first electrodes and said respectiveresistors.
 4. Method according to any of claims 1 to 3, characterised inthat said first connections (28) of a same sensor are arranged such thatthey are cut when the sensor is diced and such that its first electrodesare then electrically insulated from each other.
 5. Method according toany of the preceding claims, characterised in that said resistors (38)and said first electric connections are arranged in thin essentiallyhorizontal film(s) on the side of the upper face (16) of said wafer. 6.Method according to any of claims 1 to 4, characterised in that saidfirst electric connections are formed at least partially of paths (72)located on the lower face (66) of said wafer and of vias (70) passingthrough the wafer.
 7. Method according to claim 6, characterised in thatsaid resistors are all arranged at least partially at said vias. 8.Method according to claim 6 or 7, characterised in that said sensors areeach formed of a two-dimensional pixel matrix.
 9. Method according toany of the preceding claims, characterised in that electric contact pads(38; 78) for connecting said first electrodes are formed between thelatter and the respective resistors and that; during said step C, saidfirst electrodes are insulated from each other.
 10. Method according toany of the preceding claims, characterised in that the values of saidresistors are provided to be substantially equal, to define asubstantially equal impedance of the pixels, allowing an absorptionlayer (20) to be uniformly deposited on these pixels by anelectrochemical method.